1. Field of the Invention
This invention relates to the field of photonics, and in particular to a method of making photonic devices, such as multiplexers (Mux) and demultiplexers (demux), and a photonic device having a novel structure.
2. Description of Related Art
The manufacture of integrated optical devices such as optical multiplexers and demultiplexers requires the fabrication of silica waveguides from low refractive index buffer and cladding silica layers and from a high refractive index core silica layer over a silicon wafer. The buffer, core and cladding silica layers must have excellent optical transparency in the 1.50 μm S-band, the 1.55 μm C-band and the 1.60 L-band for use as effective photonic devices.
FIGS. 1 and 2 taken from our U.S. co-pending patent application Ser. No. 09/833,711, the contents of which are incorporated herein by reference, clearly demonstrate that the novel PECVD technique described and claimed therein results in low refractive index buffer and cladding silica layers free of the undesirable residual SiN—H and Si:N—H oscillators, which show up as an FTIR peak centered at 3380 cm−1 and whose second harmonic causes optical absorption in the 1.55 μm C-band.
FIG. 3 taken from our co-pending U.S. patent application Ser. No. 09/867,662, the contents of which are incorporated herein by reference, clearly demonstrates that the novel PECVD technique described and claimed therein results in a high refractive index core silica layer also free from the undesirable residual SiN—H and Si:N—H oscillators.
FIG. 4 taken from our co-pending U.S. patent application Ser. No. 09/956,916, the contents of which are herein incorporated by reference, demonstrates that a high refractive index core silica layer free from the undesirable residual SiN—H and Si:N—H oscillators can be achieved after a thermal treatment of only 600° C.
FIG. 5 shows the residual infrared optical absorption performance of optical waveguides fabricated from the combination of a buffer, core and cladding layer and following a thermal treatment at either 600° C. or 800° C. so as to completely eliminate the residual SIN—H and Si:N—H oscillators of the silica layers. The residual optical absorption of the waveguides treated at 800° C. is mainly limited by light scattering at the various vertical and horizontal interfaces between the core and its surrounding cladding and buffer layers. This residual light scattering can be reduced if the roughness of these vertical and horizontal interfaces can be reduced. This reduction of the vertical interfaces can be achieved by using special masking and deep-etch techniques so as to provide smooth side-walls.
Hitachi, Ltd. (Tokyo, Japan) U.S. Pat. No. 4,896,930, the contents of which are herein incorporated by reference, describes a series of process steps that can be used to prepare a substrate which has a hollow part intended to form an optical waveguide by charging an organic nonlinear optical material. Electrodes and other circuit elements are formed on the surface of a silicon substrate and followed by a SiO2 layer either deposited by CVD or grown by thermal oxidation. An opening having a width narrower than that of the required optical waveguide is etched in the SiO2 layer at a predetermined position. An isotropic etch of the silicon substrate is then performed using the SiO2 layer as a hard mask, thus providing a channel having a width larger than that of the opening of the SiO2 layer. Thermal oxidation of the silicon substrate sidewalls of the formed channel allows the formation of a SiO2 layer. A SOG layer is then coated on the surface and heat treated at a temperature of about 450° C. or more as to form a SOG layer that closes the opening without entering into the opening, thus forming a hollow part. Finally, an optical waveguide can then be formed by loading an organic non-linear optical material in the molten state into the hollow part by capillary action or vacuum suction.
France Telecom U.S. Pat. No. 5,291,574 describes a method of making strip optical waveguides based on polymer materials on a gallium arsenide (GaAs) substrate This method involves spin coating a lower layer of low refractive index SOG polymer buffer (n=1.40) followed by its curing at about 450° C.; spin coating a high refractive index core polymer, such as polymethylmethacrylate (n=1.49), containing active molecules that can be oriented by exposing the core polymer to a corona discharge at a temperature close to about 100° C., thus providing electro-optic properties to the core polymer; depositing a 0.15 μm thick silicon nitride (Si3N4) layer followed by its patterning using a photoresist based photolithography with a CF4 plasma; selective etching of the high refractive index electro-optic core polymer (using an oxygen plasma and the silicon nitride layer as hard mask) until the lower buffer polymer layer is exposed; etching of the silicon nitride hard mask layer protecting the core polymer; and depositing a low refractive index cladding polymer onto the resulting structure followed by a cure at about 100° C.
The resulting optical waveguides have poor optical performance, with a reported value of 2 dB/cm. This is not at all surprising considering the low temperature of the thermal treatments of the various layers.
Kyocera Corporation (Kyoto, Japan) U.S. Pat. No. 5,972,516 describes a method of manufacturing a single mode silica-based optical waveguide using a 7 μm thick and 7 μm wide core having a refractive index 0.25% higher than its overlaying cladding. The 7 μm thick core layer can be fabricated by: a CVD method, a FHD (Flame Hydrolysis Deposition) method, a vacuum evaporation method, a sputtering method, and a SOG (spin on glass) method using alkoxy silane as a starting material. The propagation loss can be reduced if the surfaces of this core layer have a roughness (Ra) of at least 1/10 or less of a light source wavelength to be used. The SOG method of producing the core layer merits attention since it allows the formation of smooth surfaces, low temperature film formation in a short time and low cost. In this method, the refractive index of the SOG-based core layer is controlled by a known amount of a metal alkoxide added to the siloxane-containing polymer SOG solution to be spun. However, the SOG method of producing the core layer is difficult to use since the required thickness of several μm causes two problems: cracking, from the associated large volume shrinkage, and birefringence, from the associated large internal stress.
Kyocera Corporation (Kyoto, Japan) U.S. Pat. No. 6,088,492 describes a technique that is similar to the previous U.S. Pat. No. 5,972,516.
None of the prior art references describes a satisfactory technique that allows optimization of the interface micro-roughness of the underlying PECVD optical layers.